The present invention relates to programmable current damping networks, and more particularly, to a programmable current damping network for damping overshoot and undershoot ringing effects in a write circuit of a magnetic disk drive.
Magnetic disk drives are employed to store large quantities of information in bits encoded on tracks of a disk as a series of logical ones and zeros. These logical ones and zeros are represented in bit cells, which are areas of near uniform size along the length of a track of the disk. It is desirable that the information bits be encoded on the disk as densely as practical, so that a maximum amount of information may be stored. This can be achieved by increasing bit cell density of the disk, namely by reducing the size of bit cells along a particular track or within a zone of tracks, thereby increasing the number of bit cells on the track. Increasing the number of bit cells per track increases the number of bit cells that can be encoded on each track, and therefore increases the amount of information stored.
Conventionally, logical ones are recorded as transitions in magnetic flux on a magnetic disk for the given bit cell; the absence of a transition indicates a logical zero. These transitions are created by switching the write current polarity through the write head. Transitions representing logical ones are preferably placed within each bit cell near the center of the bit cell, so that the data frequency (based on bit cell size and rotational speed of the disk) can be accurately locked by a phase locked loop during recovery of data from the disk, and to ensure that bits are not encoded over a bit cell boundary during write operations. As bit cells are more densely packed on a track, placement of transitions becomes even more important and difficult to precisely control. Thus, transition placement accuracy and bit cell density are two very important parameters in a write circuit for a disk drive.
Due to the inductive nature of a write circuit head and the parasitic capacitance associated with the write circuitry, overshoot and undershoot effects occur in the write current signal which delay the settling of write current to its final DC value. These overshoot and undershoot effects, sometimes called "ringing", adversely affect the transition placement and bit cell size concerns. One option when overshoot and undershoot effects are present is to simply wait for the write current to settle to its final DC value, and then enable the next transition for encoding a bit. This option means that bit cell duration must be increased to allow time for the write current to settle. While the accuracy of transition placement within bit cells in such a system would not be negatively affected by the ringing of the write current, the density of bit encoding by the write circuit is poor in comparison to desired goals. Another option when overshoot and undershoot affects the placement of the transition is to switch the write current before it has settled to its final value. This approach may meet acceptable encoding density, but results in decreased placement accuracy of the transitions and hinders subsequent recovery of data from the disk. More particularly, if the write current has not fully settled from a prior transition, switching for the next transition might commence in different or uncontrolled current levels, which results in sporadic placement of transitions in bit cells. Therefore, both options entail undesirable performance trade-offs where overshoot and undershoot occur.
One known solution to overshoot and undershoot problems is to connect a damping resistor across the terminals of the write head. The resistive damping technique reduces the settling time for the write current signal flowing through the head. However, resistive damping has several negative effects on the performance of the write circuit. Since some of the write current is diverted through the damping resistor, write current through the head is reduced. To achieve the desired value of write current through the head, more current must be generated to flow through both the head and the damping resistor. The damping resistor also slows the rise time for write current transitions. This can adversely affect the bit placement and cell density. While resistive damping reduces settling time, slower rise times may not be acceptable for high performance write circuits. Additionally, undershoot, which causes the loss of saturation of the head media, contributes to the problem of switching from uncontrolled current levels, and results in sporadic bit placement in the bit cells. Thus, there is need for a programmable damping network and disk drive write circuitry which overcomes the problems with undershoot while providing the user with a means to effectively control overshoot.